Ionized gas or “plasma” may be used during processing and fabrication of semiconductor devices, flat panel displays and other products requiring etching or deposition of materials. Plasma may be used to etch or remove material from semiconductor integrated circuit wafers, sputter or deposit material onto a semiconducting, conducting or insulating surface. Creating a plasma for use in manufacturing or fabrication processes typically is done by introducing a low-pressure process gas into a chamber surrounding a work piece such as an integrated circuit (IC) wafer. The molecules of the low-pressure gas in the chamber are ionized into a plasma by the radio frequency energy (power) source after entering the chamber, and the plasma flows over the work piece. The chamber is used to maintain the low pressures required for the plasma and to serve as a structure for attachment of one or more radio frequency energy sources.
Plasma may be created from a low-pressure process gas by inducing an electron flow that ionizes individual gas molecules by transferring of kinetic energy through individual electron-gas molecule collisions. Typically, electrons are accelerated in an electric field such as one produced by radio frequency (RF) energy. This RF energy may be low frequency (below 550 KHz), high frequency (13.56 MHz), or microwave frequency (2.45 GHz).
The two main types of etching in semiconductor processing are processing includes plasma etching or reactive ion etching (RIE). A plasma etching system includes a radio frequency energy source and a pair of electrodes. A plasma is generated between the electrodes, and the work piece (i.e., substrate or wafer) to be processed is arranged parallel with one of the electrodes. The chemical species in the plasma are determined by the source gas (es) used and the desired process to be carried out.
A problem that has plagued prior art plasma reactor systems is the control of the plasma to obtain uniform etching and deposition. In plasma reactors, the degree of etch or deposition uniformity is determined by the design of the overall system, and in particular the power control of the electrodes used to create the plasma in the interior of the reactor chamber.
In a plasma reactor system, at least one electrode is connected to an RF power supply. The technological trend in plasma reactor design is to increase the fundamental RF driving frequency of the RF power supply from the traditional value of 13.56 MHz to 60 MHz or higher. Doing so improves process performance, but increases the complexity of reactor design. A second trend in reactor design is to have multiple or multi-segmented electrodes. However, segmented electrodes combined with increased operating frequencies makes the delivery of the correct amount of RF power more complicated because of capacitive coupling and greater sensitivity to parasitic capacitive and inductive elements. This effect is exacerbated by the shorter wavelengths of higher fundamental frequencies. The result is increased difficulty in improving process uniformity.
Power delivered to a multiple segment electrode presents a unique power control problem. Each segment of the electrode acts as both a transmitter element and a receiver element. If each segment is powered at the same RF frequency, differentiation of received power, or reflected power, and transmitted, or forward, power, becomes difficult. This is because conventional phase and magnitude detectors cannot differentiate between power emanating from the power supply and power transmitted through the plasma from another power supply and received by the electrode.
Compensating for reflections in an RF power deliver system for a plasma reactor having a segmented electrode also requires accurate impedance measurements. These impedance measurements are needed to adjust the parameters of the matching network. However, forward and reverse propagating energy render conventional measurements difficult to interpret.
There are several U.S. patents related to plasma processing systems and apparatus and control of power thereto. For example, U.S. Pat. No. 5,556,549, entitled “Power control and delivery in plasma processing equipment,” describes an invention that monitors the power, voltage, current, phase, impedance, harmonic content and direct current bias of the radio frequency energy being delivered to a plasma chamber. In addition, the plasma mode of operation may be controlled by creating either a capacitively or inductively biased radio frequency source impedance. A radio frequency circulator prevents reflected power from the plasma chamber electrode from damaging the power source and it further dissipates the reflected power in a termination resistor. The termination resistor connected to the circulator also effectively terminates harmonic energy caused by the plasma non-linearities. Multiple plasma chamber electrodes and radio frequency power sources may be similarly controlled. However, a shortcoming of this invention is that it concentrates on a single electrode implementation and does not address the complex issue of power control and delivery to multiple electrodes. The invention also does not teach one skilled in the art how to differentiate between power reflected back to an RF power source due to load impedance mismatch and power received from an adjacent electrode. Rather, the invention describes the use of conventional phase and magnitude detectors for power control. Such detectors may not function correctly in multiple electrode/multiple match network configurations.
U.S. Pat. No. 5,889,252, entitled “Method of and apparatus for independently controlling electric parameters of an impedance matching network,” describes an arrangement and method for matching a load and a power source, such as an RF power source for a vacuum plasma processing chamber, and includes a match network coupled between the power source and the load. The match network has at least two controllably variable electrical characteristics. A sensor is provided that senses at least two parameters of the load. A drive controller responds to the sensed parameters of the load to independently control variation of a first one of the electrical characteristics of the match network as a function of only one of the parameters of the load, and a second one of the electrical characteristics of the match network as a function of another one of the parameters of the load. This is done until the power source and the load are in a matched condition. The separation of the match variables to establish a nearly one-to-one correspondence with the load parameters allows independent adjustment of the match variables to provide fast and unambiguous reaching of the matched condition. However, a shortcoming of this invention is that it does not account for a configuration having multiple electrodes (i.e., electrode segments) and the associated multiple RF power supplies and match networks. Accordingly, the invention does not teach how to independently control independently driven electrodes.
U.S. Pat. No. 5,733,511, entitled “Power distribution for multiple electrode plasma systems using quarter wavelength transmission lines,” describes a multiple electrode plasma reactor power splitter and delivery system to provide balanced power to a plurality of powered electrodes by utilizing the properties of quarter wave length transmission lines. Each electrode is supplied power by a separate (2N+1)λ/4 wavelength cable, where N=0, 1, 2. . . , connected to a common point at a load matching network's output. The impedance transformation properties of these lines are also employed to convert the plasma load to one that is more efficiently matched . Also disclosed is a technique of splitting a single large active electrode into smaller active electrodes powered by the above distribution scheme in order to achieve maximum uniformity of the reactive plasma throughout the working volume. However, a shortcoming of this invention is that it does not teach how to independently control a plurality of RF power supplies to independently drive a corresponding plurality of electrode segments.
U.S. Pat. No. 5,140,223, entitled “Circuit for adjusting the impedance of a plasma section to a high-frequency generator,” describes a circuit for adjusting the impedance of a plasma section to a high-frequency generator wherein three capacitors are connected in series between the high-frequency generator and an electrode of the plasma section; located between the generator and the electrode are two parallel oscillatory circuits. However, a shortcoming of this invention is it does not teach how to independently control a plurality of RF power supplies to independently drive a corresponding plurality of electrode segments.